1. Field of the Invention
The present invention relates to a color laser printer, and more particularly, to a color laser printer which produces a color image using a single laser scanning unit.
2. Description of the Related Art
Monochrome laser printers use a single laser scanning unit (LSU) and a single organic photoconductive cell (OPC) drum since they transfer only black ink onto a sheet of paper. In contrast, color laser printers need four LSUs and four OPC drums to transfer four ink colors, such as black (B), magenta (M), yellow (Y), and cyan (C), onto the sheet. As shown in FIG. 1A, a color laser printer includes a toner container 104, OPC drums 100-K, 100-C, 100-M, and 100-Y for black, cyan, magenta, and yellow, respectively, LSUs 102-K, 102-C, 102-M, and 102-Y forming electrostatic latent images by scanning laser beams over the OPC drums 100-K, 100-C, 100-M, and 100-Y electrically charged to a predetermined potential, developing units 105-K, 105-C, 105-M, and 105-Y developing the electrostatic latent images with four color developing solutions, a transfer belt 108 receiving developed images developed on the OPC drums 100-K, 100-C, 100-M, and 100-Y, a transfer unit 110 transferring a four color image formed of the developed images superimposed on the transfer belt 108 onto a paper sheet P, and a fixing unit 115 fixing the transferred image on the paper sheet P by applying heat and pressure to the paper sheet P.
As described above, to produce the color image, the conventional color laser printer uses the OPC drums 100-K, 100-C, 100-M, and 100-Y for black, cyan, magenta, and yellow, respectively, and the four LSUs 102-K, 102-C, 102-M, and 102-Y corresponding to the four colors.
Each LSU 102-K, 102-C, 102-M, or 102-Y scans a laser beam over a photoconductive medium, such as the corresponding OPC drum 100-K, 100-C, 100-M, or 100-Y to form an electrostatic latent image. Referring to FIG. 1B, a typical LSU includes a light source 107 emitting the laser beam, a rotary polygon mirror 109 driven by a motor (not shown) and reflecting the laser beam emitted from the light source 107, an f-θ lens 115 focusing the laser beam reflected by the rotating polygon mirror 109 on a surface of a photoconductive drum 110, e.g., one of the OPC drums 100-K, 100-C, 100-M, and 100-Y, to form a spot of an appropriate diameter along a scanning line 118, and a reflector 120 disposed on an optical path between the f-θ lens 115 and the photoconductive drum 110 to reflect an incident beam so that the laser beam passing through the f-θ lens 115 is directed toward the photoconductive drum 110. A predetermined electrostatic latent image is formed on the photoconductive drum 110 by controlling an on/off state of the light source 107.
Furthermore, a collimating lens 122 converting the laser beam into a parallel beam to an axis of the collimating lens 122, and a cylindrical lens 135 converging the parallel beam to a reflective surface of the rotary polygon mirror 109 are disposed along the optical path between the light source 107 and the rotary polygon mirror 109. A sensor 125 is disposed to detect a position where the scanning line 118 starts.
Here, the laser beam emitted from the light source 107 is converted into the parallel beam by the collimating lens 122, and the parallel beam passes through the cylindrical lens 135 and is reflected by the rotary polygon mirror 109. The beam reflected off from the rotary polygon mirror 109 passes through the f-θ lens 115, and the reflector 120 changes the optical path of the beam so that the beam is focused on the photoconductive drum 110 to form the spot at a point along the scanning line 118 of the photoconductive drum 110.
The color laser printer configured as described above has not yet become popular due to high manufacturing costs. A manufacturing cost of the LSUs is a largest percentage of a total cost of the color laser printer. Therefore, one way to reduce the cost of the color laser printer is to minimize the number of the LSUs used therein.
Another problem with this type of color laser printer is that a jitter and a periodic oscillation occur when a spindle motor drives the rotary polygon mirror for each color, and vary according to each LSU. This causes four different color lines to be printed along different scanning lines when a user desires a single line to be printed on a paper sheet, thus lowering a printing quality. To solve these problems, conventional color laser printers require an electrical, mechanical, or optical adjustment for the jitter and the periodic oscillation. However, this process increases the manufacturing costs.
Most recently, in order to reduce the manufacturing costs, research on minimizing the number of the LSUs has been conducted. An example of conventional color laser printers having a single LSU is disclosed in Japanese Laid Open Patent Application No. 2000-43333A. Referring to FIG. 2, this conventional color laser printer consists of a spindle motor 142, a rotating polygon mirror 143, and first through fourth laser diodes 140a–140d arranged in parallel so that a beam emitted from each of the diodes 140a–140d strikes the rotating polygon mirror 143 at a different incidence angle.
In the color laser printer configured as described above, four beams are reflected through the rotating polygon mirror 143 at different angles, which requires an f-θ lens 145 of a large thickness. However, as is well known, f-θ lenses of the large thickness are more difficult to manufacture.
Furthermore, the beams reflected off from the rotating polygon mirror 143 have different focal distances with respect to reflectors 144a–144d, through which each beam is respectively reflected, and with respect to first through fourth photoconductive drums 147a–147d on which each beam is respectively focused. Due to their different focal distances, a spot for each color is formed at a different position on each of the photoconductive drums 147a–147d, and therefore, first through fourth correcting lens 146a–146d are separately provided to prevent the different focal distances, which cause image differences, and to make the image differences equal or corrected. Overall, this construction increases the manufacturing and assembly costs. Further, compared with the color laser printer using the four LSUs, this color laser printer has a limited advantage in terms of optical characteristics.
Another example of conventional color laser printers is disclosed in U.S. Pat. No. 6,061,079. Referring to FIG. 3A, this color laser printer has a similar configuration in some respects with the Japanese Laid Open Patent Application No. 2000-43333A described above. That is, the color laser printer includes a spindle motor 152, a rotating polygon mirror 153, and first through fourth laser diodes 150a–150d arranged in parallel (i.e., sequentially in a longitudinal direction) so that a beam emitted from each of the diodes 150a–150d is incident upon the rotating polygon mirror 153 at a different angle.
In the color laser printer shown in FIG. 3A, a distance between an f-θ lens 155, through which four light beams reflected by the rotating polygon mirror 153 pass, and each of first through fourth photoconductive drums 160a–160d for different colors on which the beams are focused, is equal. As shown in FIG. 3B, each of first through fourth light beams I, II, III and IV, reflected by the f-θ lens 155 along different paths, is reflected through a prism-type reflector 156 and focused near each of the photoconductive drums 160a–160d corresponding to each color through each of cylindrical lenses 158a–158d. 
However, one drawback of the color laser printer configured as described above is that a thickness of the f-θ lens 155 becomes large since each of the four light beams I, II, III and IV passes through the f-θ lens 155 at different vertical positions. As described above, this makes it difficult to manufacture the f-θ lens 155, thereby increasing the manufacturing costs. Another drawback arises from the fact that separate cylindrical lenses 158a, 158b, 158c, and 158d are used to focus the four light beams near the photoconductive drums. Since the four cylindrical lenses 158a, 158b, 158c, and 158d cause different amounts of aberration and lead to an assembly error when combined together. As a result, despite the use of only one LSU, a single line command results in four color lines printed along different scanning lines, thus lowering the printing quality.